The advent of the AIDS virus requires that structures of the greatest safety be employed for hypodermic needle syringes. While blood extraction for diagnosis is an important area of hypodermic needle contamination, a much more frequent use of hypodermic needles occurs in the injection of medicine, anesthesia, and various liquids. Once an injection is made and the needle withdrawn from the patient, the needle tip is contaminated with the microorganisms of the patient. Thereafter, any accidental pricking of another person by the needle will transfer to that person such microorganisms. Various sliding sheaths have been devised that remain mechanically connected to the syringe at all times. Usually, these sheaths are spring-biased to cover the needle. However, if the sheath contacts a person it will retreat and slide until the needle penetrates that person, passing on the contamination. The contamination is injected under the skin, an area where it cannot be easily removed and where the contamination is most likely to spread. For this reason, latches of various types have been used to hold the sheath in its protective position over the needle.